1. Field of the Invention
The present invention relates in general to a line tripler for a national television system committee (NTSC)/high definition television (HDTV) dual receiver.
2. Description of the Prior Art
For the purpose of facilitating a better understanding of the present invention, sizes of HDTV and NTSC screens will hereinafter be mentioned before description of the prior art. The size of the active NTSC screen is 720.times.240 as shown in Fig. 1a, where 720 is the number of horizontal samples and 240 is the number of vertical lines in an interlaced scanning system. The number of the vertical lines is 480 in a non-interlaced scanning system. The size of the active HDTV screen is 1280.times.720 as shown in Fig. 1b, where 1280 is the number of horizontal samples and 720 is the number of vertical lines.
Generally, a HDTV/NTSC dual receiver has a construction more suitable to reception of a HDTV signal than that of a NTSC signal. For this reason, upon receiving the NTSC signal, the HDTV/NTSC dual receiver must convert a format of the received NTSC signal into that of the HDTV signal for display. Namely, the HDTV/NTSC dual receiver must convert 720.times.240 pixels of the NTSC signal into 1280.times.720 pixels of the HDTV signal. For conversion of the format of the NTSC into that of the HDTV signal, the HDTV/NTSC dual receiver uses directly a memory device of an NTSC receiver to store the NTSC signal and then converts the format of the NTSC signal stored in the memory device into that of the HDTV signal utilizing a conversion circuit. Such a conventional conversion circuit may be provided in a DSC-HDTV system available from Zenith Co., Ltd, U.S.A., and is shown in a block form in FIG. 2, herein.
As shown in FIG. 2, the conventional conversion circuit comprises a line doubler 1 for converting a 720.times.240 aspect ratio of an NTSC video signal into a 720.times.480 aspect ratio, and an up-converter 2 for converting the 720.times.480 aspect ratio of the NTSC video signal from the line doubler 1 into a 1280.times.720 aspect ratio of an HDTV format. For conversion of the 720.times.240 aspect ratio of the NTSC video signal of the interlaced scanning system into the 1280.times.720 aspect ratio of the progressive HDTV format as mentioned above, the vertical lines are tripled and 560 samples must be added in the horizontal direction of the screen.
In the conversion circuit of FIG. 2, the number of the vertical lines of the NTSC video signal is first doubled from 240 to 480 by the line doubler 1 and then increased from 480 to 720 by the up-converter 2. As a result, the format of the NTSC video signal is converted into the HDTV format in the vertical direction of the screen. On the other hand, the up-converter 2 removes a noise of the NTSC video signal through its internal filter and then increases the number of the horizontal samples of each line of the NTSC video signal from 720 to 1280 at the same time as the output of the 720 vertical lines. The increase in the number of the horizontal samples of the NTSC video signal is enabled by a sampling rate controlled by the up-converter 2. As a result, the format of the NTSC video signal is converted into the HDTV format in the horizontal direction of the screen.
However, the above-mentioned conventional conversion circuit has a disadvantage in that the up-converter is complex in construction. This makes the whole construction of the HDTV/NTSC dual receiver relatively complex. Also, the line doubler is high in price, resulting in an increase in the manufacturing cost of the HDTV/NTSC dual receiver.